Cleaning of crankcase gas requires a cleaning device, which can effectively separate very small particles, solid and/or liquid, suspended in the crankcase gas. Different types of cleaning devices have been proposed, such as traditional filters, cyclones or different kinds of centrifugal separators having rotating members. Lately, centrifugal separators of a relatively advanced kind have been proposed for such cleaning, and different methods have been proposed for driving of centrifugal separators of these kinds. Thus, it has been suggested that a centrifugal separator for this purpose should be driven mechanically by means of one of the ordinary shafts of the combustion engine, e.g. the crank shaft or the cam shaft (see for instance U.S. Pat. No. 5,954,035). Another suggestion has been that a centrifugal separator should be driven by means of an electric motor (see for instance WO 01/36103). A further suggestion has been that a fluid, gas or liquid, should be pressurized by means of the combustion engine and be used for driving of a turbine of one kind or another, coupled to a centrifugal rotor for cleaning of crankcase gas (see for instance WO 99/56883).
Irrespective of the kind of device used for cleaning of crankcase gas that is produced by a combustion engine it is difficult to avoid that the operation of the cleaning device influences the crankcase gas pressure prevailing in the crankcase of the combustion engine. Either the cleaning device creates a counter pressure for the crankcase gas to be cleaned, which may lead to an undesired overpressure in the crankcase, or the cleaning device causes a certain underpressure in its inlet for crankcase gas to be cleaned, which may propagate to the crankcase of the combustion engine and created therein an undesired underpressure. In many cases there is a requirement that the pressure in the crankcase must be kept within a certain pressure interval, i.e. it must not-rise above a certain first value and must not drop below a certain second value during the operation of the combustion engine.
The said problem may theoretically be solved for instance by means of pressure sensing control valves of different kinds. However, great demands are put on the pressure sensitivity of such valves, and these demands may be difficult to fulfil in the environment where the valves have to operate.
The object of the present invention is to provide a method and a device for cleaning of crankcase gas generated during operation of a combustion engine in its crankcase, by means of which a predetermined gas pressure, or a gas pressure within a predetermined pressure interval, is maintained in the crankcase of the combustion engine.